1. Field of the Invention
The invention relates to a tool consisting of a base material selected from the group consisting of cemented carbide, cermet, hard material and tool steel and a coating comprising two or more layers with a total coating thickness of from 0.5 μm to 15 μm, a coating layer having a thickness of from 0.0003-5.0 μm.
Cemented carbides, cermets, hard materials and tool steels are used for tools which are exposed to high wearing stresses. The term cemented carbide is to be understood as meaning a composite material which is composed of a hard material phase and a metallic binder. The cermet group of materials comprises all materials which are composed of one or more ceramic phases and one or more metallic phases. Hard materials encompass all materials with a hardness of >1000 HV. These include compounds of elements from group IVa to Vla of the periodic system with the elements carbon, nitrogen, boron or silicon. However, this group of materials also encompasses diamond, cubic boron nitride, silicon carbide, sialons, aluminum oxide, aluminum nitride and silicon nitride, to mention just the most important. Tool steels in accordance with DIN 17 300 are steels defined by their use in tools.
To increase the resistance to wear, wear-resistant hard material coatings based on carbides, nitrides, borides, silicides and oxides are applied to the base material. These coatings may comprise one or more coating layers and have hardnesses which are usually in the range from 1500 HV to 4000 HV. By way of example, reference may be made to single-layer or multilayer coatings consisting of titanium nitride, titanium carbon nitride, titanium aluminum nitride or aluminum oxide.
Under load, the tribological system, in addition to the tool, also comprises opposing bodies which cause the wear and friction, possibly intervening materials, the forces which are active, the sequence of movements and the environmental influences. In particular if the forces which are active and the relative velocity between tool and opposing body are high, a considerable temperature rise occurs in the superficial boundary region between the wearing part and the opposing body. For example, temperatures of 1000° C. and in some cases even above are measured at the surface of a machining tool. The causes of this are deformation and separation work in the shearing zone, friction of the chip at the primary face and friction of the work piece at the flank.
Hard-material coatings based on TiAlN are in widespread use for improving the wearing properties of tools. The influence of different Al contents on the improvement to the abrasion and oxidation resistance and the influence of further elements on the abrasion resistance of titanium nitride coatings, in particular produced using PVD processes, have been investigated.
For example, Thin Solid Films, 343 (1999), pp. 242-145 has described TiAlBN layers which have been produced in an Ar plasma at 450° C. However, it was not possible to achieve any significant improvements to the wear resistance during dry machining.
Surface Coat. Technol., 133-134 (2000) pp. 145-151 has disclosed TiAlSiN coatings produced by means of cathode arc vaporization and containing from 3.8 at. % to 8.2 at. % Si, wherein case hardness values from 35 to 45 GPa were achieved.
Furthermore, in a TiAlN coating it is also possible for Ti to be substituted by Cr, Hf, Zr or Nb.
For example, international PCT publication WO 2004/059030 describes an AlCrN coating with improved wear properties.
European patent publication EP 0 558 061 describes the partial or complete substitution of the Ti in the titanium nitride by the elements Hf, Zr, Cr or Nb, hard-material coatings of this type being joined to the base material via a metallic interlayer of Nb or Ta.
Japanese patent publication JP 2003-034859 describes a wear-resistant MeAlCrVCN coating, wherein at least one of the elements Ti, Nb, W, Ta or Mo is used for Me.
European patent publication EP 1 201 776 describes a gear-cutting tool made from high-speed steel with a multilayer coating combination comprising TiAlTaN or TiAlTaCN and TiAlN or TiAlCN, which is used for the machining of low-alloy steels or soft steels. The combination of the two layers results in a high service life combined with a low susceptibility to chip sticking.
In the event of high tool stresses, as occur for example in the dry milling of alloyed steels, very high surface temperatures are reached in the tool contact zones, such as for example the primary face and flank. Similarly high stresses also occur, for example, in the entry region of a shaping tool. The wear resistance of the known coating systems described above has hitherto been unsatisfactory in these applications, in particular if coolants are not used.